It is commonplace in the electronic art to combine a modulated or modulating signal with a local oscillator signal in order to obtain a further modulated signal at another frequency that is more easily amplified, filtered, broadcast, and/or detected. This is done in a mixer.
In a typical demodulation application, a modulated radio frequency (RF) signal is combined in a mixer with a local oscillator (LO) signal to produce an intermediate frequency (IF) signal which may be then further amplified and detected to recover the information modulated onto the RF carrier. Alternatively, this process can be reversed, mixing an LO signal with an IF signal to produce a modulated carrier (RF) signal for further amplification and ultimate transmission as a modulated output signal.
The demodulation mixing process produces sum and differences of the RF and LO frequencies. One or more of the sum and difference frequencies is at the desired IF frequency, according to the following relations:
(1) f.sub.IF =f.sub.LO -f.sub.RF, i.e., down conversion where f.sub.LO &gt;f.sub.RF, PA1 (2) f.sub.IF =f.sub.RF -f.sub.LO, i.e., down conversion where f.sub.LO &lt;f.sub.RF, PA1 (3) f.sub.IF =f.sub.LO +f.sub.RF, i.e., up conversion.
Similar relations apply to modulation of a carrier signal.
Examination of equations (1) and (2) shows that there is not a unique correspondence between f.sub.LO, f.sub.IF, and f.sub.RF. For a given value of f.sub.LO, two different values of f.sub.RF may produce the same value of f.sub.IF. For example, (see FIG. 1) for f.sub.LO =3 GigaHertz, both f.sub.RF1 =2.5 GigaHertz and f.sub.RF2 =3.5 GigaHertz can produce f.sub.IF =0.5 GigaHertz. The RF and IF frequencies are generally not discrete frequencies but narrow frequency bands determined by the modulation thereon. The LO frequency is typically sharply defined, but may be time varying in some cases.
A prior art double balanced mixer apparatus 10 is illustrated in FIG. 2. Mixer apparatus 10 comprises input 12, input 42, output 15, balun transformers 16, 17, and four port mixer element 19 comprising diodes 24, 25, 27, 29, and having input ports 9, 23, 26, 28. RF input signal 14 comprising either or both RF1 and RF2 (FIG. 1) enters at RF port 12. Balun transformer 16 splits incoming signal 14 into two substantially equal amplitude RF signals 18, 20 which have approximately a relative phase displacement of 180.degree.. Signal 18 is sent to port 23 of four port mixer element 19 and signal 20 is sent to port 28 of mixer element 19. Similarly, LO input 11 supplies LO signal 13 to balun transformer 17. Balun transformer 17 splits LO signal 13 into two substantially equal amplitude RF signals 21, 22 having approximately a 180.degree. relative phase displacement. Signal 21 is sent to port 26 of four port mixer element 19 and signal 22 is sent to port 9 of four port mixer element 19.
The nonlinear current versus voltage characteristics of diodes 24, 25, 27, 29 cause signals to be created at frequencies in accordance with equations 1-3, which signals are coupled to IF port 15. Because balun transformers 16, 17 must be able to pass the RF, LO, and IF frequencies, the required bandwidth of the balun transformers is more difficult to realize.
Furthermore, balun transformers such as 16, 17 are generally most useful at frequencies below about one GigaHertz. This limits the frequency range over which prior art mixer apparatus 10 is useful. Alternatively, hybrid devices have been used instead of balun transformers to split signals into two substantially equal amplitude signals having a 180.degree. relative phase difference. These 180.degree. hybrids are typically achieved typically using lumped or semi-lumped networks of capacitors, inductors and resistors. Examples of lumped element hybrids are given in U.S. Pat. Nos. 4,992,761, 5,023,576 and 5,045,821 assigned to the assignee of the present invention. These hybrids have a number of disadvantages at frequencies above several GigaHertz which include excessively small component size and cross-coupling problems making prior art hybrids unsuitable in very high frequency monolithic applications unsuitable.
Alternatively, "magic-tees" have been used as balun transformers to split signals into two substantially equal amplitude signals having a 180.degree. relative phase difference. Magic tees are typically implemented in waveguide form and provide improved isolation between the output ports and between the input ports. The improved isolation is desirable for mixer applications, for example where the RF and LO frequency bands overlap. These waveguide devices are well known in the art.
Mixers are alternatively employed for modulation of an LO signal by an IF signal to produce a modulated carrier, or RF signal. This process is similar to the demodulation process described above, with LO port 11 and IF port 15 accepting input signals and RF port 12 providing an output signal.
Prior art mixers have a number of disadvantages well known in the art. Among these disadvantages are, for example: (1) inadequate port-to-port isolation, (2) limited bandwidth, particularly intermediate frequency bandwidth, (3) relative complexity and (4) difficulty of implementation in compact form suitable for incorporation in monolithic microwave integrated circuits (MMIC's).
MMIC's are typically constructed using Si, GaAs, or other compound or elemental semiconductor integrated circuit (IC) wafer processing technology on and/or in such wafers. It is highly desirable to have broadband mixers which can be made with lumped elements or other structures that are compatible with IC fabrication techniques and geometries. In particular, it is important that they be of comparatively small size so as to not occupy disproportionately large substrate areas compared to the semiconductor diodes, transistors, etc., which mix the signals, or compared to the amplifiers or other signal processing elements that may be included in the MMIC.
Such concerns are important in the frequency range above one GigaHertz and especially important from about 20 to 40 GigaHertz and above where the sizes of distributed circuit elements are unwieldy. In particular, for broad-band applications, the use of hybrids or baluns requires many lumped or distributed elements which, in monolithic applications, require substantial die area. In order to achieve octave and multi-octave bandwidths, for example, several lumped element baluns must be cascaded together resulting in many components which is a major disadvantage since the die area is at a premium. Further, interactive coupling between the components may result from the additional elements in a small area degrading mixer performance.
Where high isolation between ports is additionally required in a mixer application, Magic-tee devices are not readily implementable in monolithic form because of their traditional waveguide construction, for example. Thus there is a need for mixers with the high isolation characteristics of Magic-tee signal inputs that are readily integratable in monolithic form.
Further, there continues to be a need for improved broadband mixers and methods of mixing signals that use fewer components, especially those which are easy to construct and/or which employ elements that are readily integratable in and/or on MMIC's or the like.